Syphilis, caused by Treponema pallidum, continues to be a significant public health problem and has been demonstrated to augment the spread of HIV. New means of prevention of syphilis, including vaccine development, are needed. In order to approach vaccine development in a rational way, antigens must be prioritized for examination based upon their functions in pathogenesis and immunity. The opsonization of T. pallidum by antibodies is crucial for bacterial clearance and for macrophage-mediated killing. Therefore, a recombinant expression library will be screened for clones reacting with opsonic rabbit serum (ORS). The ORS is immune rabbit serum from which many "irrelevant" antibodies (those not important for opsonization, including antibodies to the major lipoproteins and the endoflagella) have been adsorbed; full opsonic capacity is retained following adsorption. Selected clones will be prioritized for further examination based upon their lack of reactivity with non-opsonic antisera (anti-Borrelia, anti-Leptospira). Promising clones will be sequenced and their full open reading frames (ORFs) will be identified in the newly released T. pallidum genome sequence. The ORFs will be expressed as recombinant proteins and antibodies will be produced. These antibodies will be used to determine if the antigens are located on the surface of T. pallidum. The role of these molecules in attachment of T. pallidum to eucaryotic cells, an early step in pathogenesis, will be examined by testing the ability of the antibodies to block adherence in vitro. The role of the antigens in opsonization will be examined by testing the antisera for their ability to facilitate phagocytosis of T. pallidum by rabbit peritoneal macrophages. Antigens that appear to be promising, based upon in their surface localization or their role in opsonization or cytoadherence, will be tested for their ability to protect rabbits from challenge with T. pallidum. This focused approach for selecting antigens of interest, based upon functions known to be important in pathogenesis and immunity, will identify important virulence factors of T. pallidum and will identify potential vaccine candidates for further study.